Starting control for an a.c. electric motor device

ABSTRACT

An electric motor is connected to an A. C. voltage supply through a thyristor switching device and is started by the full voltage starting method. In order to prevent the thyristors in the thyristor switching device from a burn-out due to an excessive starting current, the thyristor switching device is controlled to be intermittently turned off during the starting period of the motor under the control of a thyristor temperature rise anticipation circuit. The thyristor switching device is also controlled to be intermittently turned on during the starting period under the control of a thyristor temperature fall anticipation circuit and a synchronous indicating circuit.

United States Patent Takahashi et al. 51 July 18, 1972 [54] STARTINGCONTROL FOR AN A11, 3,562,587 2/l971Forst......................................i.3l7/13 CTR] 3,569,7813/1971 Strachan... ...i.3l8/473 X ELE C MOTOR DEVICE 3,582,740 6/ I971Reinert ..3l7/l 3 R [72] Inventors: Hideoml Taltalmshl, Tokyo;Kltsuhllto Taklglml, Kawasaki, both of Japan Primary Examiner-BcnjaminDobeck 731 Assignee: Tokyo Shlbaurn Electric Co., 1211.,

Kawasaski, Kanagawa-ken, Japan I 57] ABSTRACT [22] F'led: July Anelectric motor is connected to an A. C. voltage supply [21] Appl. No.;165,559 through a thyristor switching device and is started by the fullvoltage starting method. In order to prevent the thyristors in thethyristor switching device from a bum-out due to an exces- [30] Foals"Applic'uon sive starting current, the thyristor switching device is con-July 25, 1970 Japan...................................45/64798 "Oiled beintermittently filmed durins 81mins period of the motor under thecontrol of a thyristor tempera- 52 us. Cl. ..31s/473, 318/22 1, 317/13 Rrise anticipation circuit The thyristor switching device is 51 1 1m. c1..3021. 7/09 also controlled w W intermittently wmcd on during the start-581 mu 0! Search ..3 1 8/473, 221 E, 227; 317/13 R "'8 Period the "Wimmature fall ticipation circuit and a synchronous indicating circuit.[56] References Cited UNITED STATES PATENTS 4 Chins, 2 Drawing Figures3,538,4[1 ll/l970 Knauer etal ..318/22l E t/l'; is u 6 B4 5 n as 876 l?\J k. Z2

PATENTED JUL 1 8 I812 SHEET 2 BF 2 BACKGROUND OF THE INVENTION 1 FieldOf The Invention The present invention relates to a starting control foran A.C. electric motor in which a thyristor switching device is employedfor connecting and disconnecting the motor to an A.C. voltage supply.

2. Description OfThe Prior Art Recently. a static switch or thyristorswitching device has been employed as a switch for an AC. electric motorsince the same has been found to be quite successful in achieving anarcless, noiseless and chatterless opening and closing of an electricpower circuit. While somewhat satisfactory, some difficulties havearisen in the thyristor switching device when it has been employed withan AC. electric motor device. Thus, for example, when the electric motorwas of an induction starting type such as an induction motor or asynchronous induction motor, and the same was started by a full voltagestarting method, an extremely large starting current having an amplitudeexceeding six to times that of the rate current would often rush intothe motor for a period of from several seconds to a minute.

Such a rush of current to the motor would oflen result in a burnout ofthe thyristors unless the same were made of a large enough size tosufficiently resist against the heavier load current flowingtherethrough. However, since the amplitude of the rated current of themotor is generally small relative to that of the starting current, theprovision of large scale thyristors was found to be quite expensive.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to provide a new and improved unique starting control for anA.C. electric motor in which thyristors having a relatively smallcapacity and size can be employed in a thyristor switching device forthe motor.

Another object of the present invention is to provide a new and improvedunique starting control for an AC. motor device in which a thyristorswitching device is intermittently turned off during the starting of themotor by the anticipation of a predetermined temperature rise of thethyristors in response to an excess starting current flowing through themotor.

Still another object of the subject invention is to provide a new andimproved unique starting control for an AC. electric motor device inwhich a thyristor switching device is intermittently turned on duringthe starting of the motor by the anticipation of a predeterminedtemperature fall of the thyristors in response to the tum-off of thethyristors.

Yet still a further object of the present invention is to provide a newand improved unique starting control for an A.C. motor device in which athyristor switching device is intermittently turned on during thestarting of the motor under the control of a thyristor temperature fallanticipation circuit and a synchronous indicating circuit which detectsa sychronization between the voltages across the thyristor switchingdevice when the thyristors therein are turned ofl'.

Briefly, in accordance with the present invention, the foregoing andother objects are in one aspect attained by the provision of a thyristorswitching device which is connected between an A.C. electric motor andan AC. voltage supply for connecting and disconnecting the motor fromthe A.C. voltage supply. A thyristor temperature rise anticipationcircuit is provided for intermittently turning off the thyristorswitching device by anticipating a predetermined temperature rise of thethyristors in the thyristor switching device in response to an excessivemotor starting current during the starting of the motor. A thyristortemperature fall anticipation circuit is provided for intermittentlyturning on the thyristor switching device during the starting of themotor by anticipating a predetemrined temperature fall of the thyristorsin the thyristor switching device in response to a tum-off of thethyristor switching device. A synchronous indicating circuit is alsoprovided for detecting a synchronization between the voltages across thethyristor switching device while the same is turned off, and for thenenabling the thyristor switching device to turn on in cooperation withthe thyristor temperature fall anticipation circuit.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of thisinvention will be readily obtained as the same becomes understood whenconsidered in connection with the accompanying drawings, wherein FIGS. 1and 2 show a circuit diagram of a starting control for an A.C. electricmotor in accordance with one preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS.1 and 2, an induction motor [0 is therein shown as having a statorwinding ll and a rotor 12. The stator winding ll is connected forenergization thereof to an AC. voltage supply 13 through a thyristorswitching device 14 which has a pair of thyristors l5 and 16 connectedin paral lel with oppositely conductive polarities.

For convenience of illustration, the induction motor 10 is shown asbeing of a single phase type, however, in an actual system the samewould be of three-phase type, and the thyristor switching device 14would be provided in each of the respective phase lines of the motor.Furthermore, it should be understood by way of example that the electricmotor 10 may be of the synchronous induction motor type. in addition, inac cordance with the present invention, a plurality of motors could beconnected through a set of thyristor switching devices to an A.C.voltage supply [3.

A current, which corresponds to the current flowing through statorwinding 11 of the motor 10, is taken out by a current transformer 17,and is passed through conductors 18 and 19 to a load 20 of FIG. 2 wherethe same is converted into a voltage drop in response to the inputcurrent of the motor 10.

The voltage drop across the load 20 is rectified through a diode 21, andis then applied to a thyristor temperature rise anticipation circuit 22.The thyristor temperature rise anticipation circuit 22 includes aconventional integrating circuit which has a plurality of resistanceelements 22 and capacitance elements 24 connected as shown. The outputof the integrating circuit is then applied to a capacitor 25 forcharging of the same. The voltage across capacitor 24 is then applied toan emitter electrode of a unijunction transistor 26. One base electrodeof the unijunction transistor 26 is connected to the conductor [9through a primary winding 27 of a transformer 28, and the other baseelectrode of the unijunction transistor 26 is connected to a positiveterminal of a D.C. voltage supply.

The transformer 28 has three secondary windings 29, 30 and 31. Thesecondary winding 31 is connected, through a diode 32, between a gateelectrode and a cathode electrode of a thyristor 33 which is an elementof a thyristor temperature fall anticipation circuit 34. A diode 35 isconnected across the primary winding 31 to eliminate any negative halfcycle voltages being induced in the winding 31. An anode of thethyristor 33 is connected to a positive terminal of a D.C. voltagesupply through a resistor 36, and is connected to ground through aseries combination of a reactor 37 and a capacitor 38. A cathode of thethyristor 33 is connected to ground through a capacitor 39.

The thyristor temperature fall anticipation circuit 34 also includes anintegrating circuit having a plurality of resistance and capacitanceelements 40 and 4], and the input of the integating circuit is connectedacross the capacitor 39 for enabling the same to be charged therefrom.The output of the lOlOdS U79 integrating circuit is applied to acapacitor 42 for charging the same and the terminal voltage of thecapacitor 42 is then applied to an emitter electrode of a unijunctiontransistor 43. One base electrode of the transistor 43 is groundedthrough a primary winding 44 of a transformer 45, and the other baseelectrode of the transistor 43 is connected to a DC. voltage supply.

The secondary winding 46 of the transformer 45 is connected to a primarywinding 47 of a transformer 48 which is located at the first stage of asychronous connecting circuit generally designated by a referencenumeral 49. The transformer 48 has a secondary winding 50 and theterminals thereof are respectively connected to a gate electrode of athyristor Sl through a diode 52 and to a cathode of the thyristor Adiode 53 is connected across the secondary winding 50 in order toeliminate any negative half cycle voltages induced in the winding 50.

An anode of the thyristor 51 is connected to a DC. voltage supplythrough a resistor 54, and a cathode of the thyristor 51 is groundedthrough a resistor 55. A series combination of a resistance element 56and a capacitance element 57 is connected in parallel with the resistor54, and an intermediate connection point 58 is connected to an anode ofa thyristor S9. The cathode of thyristor 59 is connected to a conductor60 which extends from the lower terminal of the secondary winding 50 ofthe transformer 48 and the cathode of the thyristor 51. The cathode ofthe thyristor 59 is grounded through a series combination of aresistance element 61 and a capacitance element 62. An intermediateconnection point of the resistance element 61 and the capacitanceelement 62 is connected to an anode of a thyristor 64, the cathode ofwhich is also grounded.

The conductor 60 is connected to a conductor 65 through a collector andan emitter of a switching transistor 66, and the same is connected to anemitter of a unijunction transistor 67 through a resistor 68. Anintermediate connection point of the resistor 68 and the transistor 67is grounded through a capaci tor 69v One base of the unijunctiontransistor 67 is connected to a DC. voltage supply, and the other baseof the unijunction transistor 67 is grounded through a primary winding73 of a transformer 74. The transformer 74 has a pair of secondarywindings 75 and 76 and the same are respectively connected to the mainthyristors l5 and 16 of FIG. I.

The secondary winding 29 of the transformer 28 is con nected to the gateof the thyristor 59 in order to cause it to turn on, while the secondarywinding 30 of the transformer 28 is connected to the gate of thethyristor 64 to cause it to turn on.

The transformers 48 and 74 respectively have primary windings 77 and 78for enabling a starting signal to be applied thereto.

In FIG. 1 the synchronous indicating circuit is generally shown with areference numeral 79, and includes a pair of transformers 80 and 81whose respective primary windings 82 and 83 are respectively connectedacross the A.C. voltage supply 13 and the stator winding ll of the motor10. The secondary windings 84 and 85 are respectively connected toZenner or constant voltage diodes 86 and 87 respectively through diodes88 and 89.

Seven NPN type transistors 90 to 96 are provided and a DC. voltagesupply 97 is provided for applying proper collector-emitter currents tothe transistors 90 to 96.

The anodes of the constant voltage diodes 86 and 87 are connected to anegative terminal of the DC. voltage supply 97, and the cathodes of thesame are respectively connected to the anodes of nonreverse diodes 98and 99 whose cathodes are grounded through serially connected resistorsI00 and 101. An intermediate connection point of the resistors I00 and101 is connected to a base of the first stage transistor 90. Thecollector voltage of the transistor 90 is applied to the base of thesecond stage transistor 91. The collector of the second stage transistor91 is connected to an intermediate connection point between a variableresistor 102 and a capacitor I03 which are connected in series acrossthe DC. voltage supply 97.

The series combination of the resistance element [02 and the capacitanceelement 103 forrm a variable timing circuit. The voltage across thecapacitor 103 is applied to the base of the third stage transistor 92.

The three transistors 93, 94 and 95 are connected in series so as toconstruct an AND gate, and the collector voltage of the third stagetransistor 92 is applied to the base of transistor 93. A voltage isapplied to the base of the transistor 94 from the cathode of theconstant voltage diode 86 through a DC. current blocking diode 104. Avoltage is applied to the base of the transistor 95 from the cathode ofthe constant voltage diode 87 through a DC. current blocking diode 105.

The output of the AND gate of the series combination of the transistors93, 94 and 95 is taken from the collector of the transistor 95 and isthen applied to the base of the final stage transistor 96. The output ofthe transistor 96 is taken from the collector thereof and then appliedthrough conductors 106 and 107 to the base and emitter of the transistor66 of FIG. 2.

In operation, in order to start the motor 10 a high frequency startingsignal is applied to the primary windings 77 and 78 of the respectivetransformers 48 and 74. The output from the secondary winding 50 of thetransformer 48 is then applied to the gate of the thyristor 51 tothereby cause it to turn on. The turn-on state of the thyristor 51 willcontinue until a gating signal is applied from the secondary winding 29of the transformer 28 to the gate of the thyristor 59. Meanwhile, thestarting signal applied to the primary winding 78 of the transformer 74will cause the main thyristors l5 and I6 in the thyristor switchingdevice 14 to turn on, and accordingly the full voltage from the AC.voltage supply 13 will be applied to the stator winding 11 of the motor10 to thereby make the motor ll) start.

Under such conditions, the voltages across the thyristor switchingdevice 14 will be completely synchronized. Thus, the transistor willturn on when the voltages of the windings 84 and 85 simultaneouslybecome positive and the second stage transistor 91 will turn off. Theterminal voltage of the capacitor 103 will increase according to the RCtime constant of the resistance element 102 and the capacitance element103 to thereby tum on the third stage transistor 92 after apredetermined time delay. As such, the transistor 93 will change itsstate from "on" to off, and the AND gate which includes the seriescombination of transistors 93, 94 and will openv To this end, the laststage transistor 96 will generate an output from the collector thereofto thereby turn on the switching transistor 66.

in this manner, the transistor 66 will connect the line 65, so that aDC. voltage is applied to the emitter of the unijunction transistor 67until the thyristor 51 is turned off. As is well known, the circuitincluding the transistor 67. the capacitor 69 and the primary coil 73 ofthe transformer 74 will provide a high frequency oscillation, and willcontinue to hold the onstate of the thyristor l5 and 16 even ifthestarting signals have been removed from transformer windings 77 and 78.

The motor current which is derived from the current transformer I7 isapplied to the thyristor temperature rise anticipation circuit 22 tothereby increase the terminal voltage across the capacitor 25 accordingto a time constant defined by the integrating circuit which includes theresistance element 23 and the capacitance element 24. Now, if thestarting rush current exceeds a predetermined value which is defined bya threshold level of the unijunction transistor 26, then the circuitwhich includes the transistor 26, the capacitor 25 and the primarywinding 27 of the transformer 28 will cause a high frequency oscillationso that output signals will be generated from the secondary windings 29,30, and 31. it can be understood that the temperature of the thyristorsl5 and 16 in the static switching device [4 will have reached an upperlimit when an excitation has been applied to the transformer 28.

Under such conditions, an output signal from the vn'nding 29 is appliedto the gate of the thyristor 59 to thereby turn it on. To this end. theelectric charges on the capacitor 57 will be discharged through thethyristor 59 to thereby negatively bias the thyristor 51 and therebyturn it off. Also, the output signal from the winding 30 will be appliedto the gate of the thyristor 64 to make it turn on. Thus, the voltagelevel of the line 60 will fall rapidly to the ground potential. Theresistors 56 and 61 serve to reduce the current flowing through thethyristors $9 and 64 to an amplitude which is less than the holdingcurrent thereof, and accordingly the thyristors 59 and 64 will benaturally turned off.

ln this way, the emitter of the unijunction transistor 67 will bedeenergized and the turn-on signal will be removed from the gates of themain thyristors l5 and 16. Thus, the static switch 14 will be turnedoff.

Meanwhile, the output signal from the winding 31 of the transformer 28will cause the thyristor 33 to turn on. In this case, the electricalcharges on the capacitor 38 will be transferred momentarily to thecapacitor 39 by the oscillation circuit which includes the capacitors 38and 39 and the reactor 37. Now, when the capacitor 39 is charged, thethyristor 31 will be naturally turned off.

It should be understood that the charges of the capacitor 39 aregradually transferred to the capacitor 42 through the integratingcircuit which includes the resistance element 40 and the capacitanceelement 41 in the thyristor temperature fall anticipation circuit 34.Now, when the terminal voltage of the capacitor 42 exceeds the thresholdlevel of the unijunction transistor 43, then the oscillating circuitwhich serially includes the capacitor 42 and the primary winding 44 oftransformer 45 will cause a high frequency oscillation to occur. It canbe understood that the aforesaid oscillation can occur at a time whenthe temperature of the main thyristors fall to a value substantiallyequal to or less than a predetermined safety value.

When the stator winding ll of the induction motor is disconnected fromthe AC. voltage supply IS, a voltage may be induced in the statorwinding ll for several hundred msec. due to a residual magnetismremaining in the rotor core. As a result thereof, if the static switch14 is turned on without any consideration of synchronization, then thestatic switch may close when the voltages thereacros are in a reversephase. This will cause an extremely large current to nish into thestator winding 1 l of the motor 10 and thereby generate an abnormallyhigh voltage which could break dowm the insulation of the motor circuit.it is thus required that sychronization be assured before the turn-on ofthe static switch 14.

The sychronizatlon between the voltages across the static switch 14according to the present invention is detected, as described hereinbelowby the synchronous indicating circuit 79.

Assuming that the voltage which appears on the secondary winding 84 ofthe transformer 80 has become positive before the voltage which appearson the secondary winding 85 of transformer 83, then the first stagetransistor 90 will turn on to thereby turn oil the second stagetransistor 9!. in accordance therewith, the timing circuit whichincludes the resistor I02 and the capacitor 103 will begin to operate.The third stage transistor 92 will still be turned off, and thetransistor 93 of the AND gate will be turned on. The transistor 94 ofthe AND gate will be turned on by the positive half cycle of the A.C.voltage supply.

The induced voltage of the stator winding 11 will soon become positiveto thereby turn on the transistor 95 of the AND gate. if the time whenthe induced voltage of the stator winding ll has become positive isreached before the turn-on of the transistor 92, which can be turned onby the predetermined voltage charged on the capacitor 103, then thetransistors 93, 94 and 95 in the AND gate will be able to be turned onto thereby turn off the final stage transistor 96. The collector voltageof the transistor 96 will then change rapidly from the ground voltagelevel to the positive voltage level of the DC. voltage supply 97.

In this way, the transistor 66 will become turned on to connect the line60 to the line 65. The circuit including the unijunction transistor 67,the capacitor 69 and the primary winding 73 of transistor 74 will againoscillate with a high frequency so as to cause the static switch 14 toturn on. The motor I0 is then synchronously connected to the A.C.voltage supply l3.

it can be seen that the serially included transistors 93, 94 and 95 ofthe AND gate will be turned ofl during a negative half cycle of the A.C.voltage supply 13. However, since one of the thyristors, which turns onduring the negative half cycle has already been turned on, the turnoffof the static switch 14 will not occur, and as soon as the next positivehalf cycle begins, the AND gate which includes the transistors 93, 94and 95 will be closed to thereby turn off the transistor 96 such thatthe turn-on state of the static switch 14 will be maintained.

it should be understood that the aforesaid operation is intermittentlyrepeated during the time when an excess starting current may be present.

When the motor speed has nearly reached the rated speed and the motorcurrent has thereby settled to the rated amplitude, then the repeatedcut-off of the motor operation by the thyristor temperature rise andfall anticipation circuits 22 and 34 will no longer oocur. Under suchconditions, the conductor 60 will be continuously energized from the DC.voltage supply through the thyristor 51 to thereby cause the staticswitch 14 to continuously turn on. It can be readily understood that themotor l0 can be deenergized by cutting off the DC. voltage supply whosevoltage is applied to the thyristor S l the unijunction transistor 67and so on.

From the above, in accordance with the teachings of the presentinvention, it can be easily understood that the static switch 14 isintermittently turned off during the starting of the motor so that anybum-out of the thyristors l5 and 16 of the static switch 14 can beavoided. With the intermittent cut-off of the static switch of thisinvention, it is now possible to use the substantially full rating ofthe thyristols l5 and 16 under the control of the thyristor temperaturerise anticipation circuit 22. Furthermore, with the use of the thyristortemperature fall anticipation circuit 34 of the present invention, it isnow possible to reclose the static switch 14 at an appropriate timewithout waste of the cooling time of the thyristors l5 and I6. Inaddition, with the use of the synchronous indicating circuit and thesynchronous connection circuit 49 of the present invention, thesynchronized connection between the motor 10 and the A.C. voltage supply13 thereof can be efl'ectively achieved during the repeated turn-off andturn-on of the static switch 14 without a reverse phase connection. Inthis way, a static switch is realized which is economical and of a smallsize.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

l. A starting control for an AC. electric motor device comprising:

an A.C. electric motor device;

an A.C. voltage supply for supplying voltage to said motor;

a thyristor switching device having thyristors therein and beingconnected between said motor device and said A.C. voltage supply;

a thyristor temperature rise anticipation circuit for intermittentlyturning off said thyristor switching device by anticipating apredetermined temperature rise of the thyristors provided in thethyristor switching device in response to an excessive motor currentduring a starting of said motor;

a thyristor temperature fall anticipation circuit for intermittentlyturning on said thyristor switching device during the starting of saidmotor by anticipating a predetermined temperature fall of the thyristorsprovided in the thyristor switching device in response to the turn-offof the thyristor switching device;

a synchronous indicating circuit for detecting synchronization betweenvoltages across the said thyristor switching device while the same isturned off, and,

a synchronized connecting circuit for enabling said thyristor switchingdevice to turn on in accordance with said thyristor temperature fallanticipation circuit.

2. A starting control for an A.C. electric motor device according toclaim 1, wherein said thyristor temperature rise anticipation circuitcomprises an integrating circuit which includes resistance andcapacitance elements therein; and,

said thyristor temperature fall anticipation circuit comprises anintegrating circuit which includes resistance and capacitance elementstherein.

3. A starting control for an A.C. electric motor device according toclaim 1, wherein said synchronized connecting circuit comprises:

a DC. voltage supply;

first circuit means being energized from said DC. voltage supply inresponse to an output fi'om said thyristor tem perature fallanticipation circuit;

second circuit means for energizing the gate of the thyristors of saidthyristor switch; and,

a switching transistor device for connecting said first and secondcircuit means in accordance with an output from said synchronousindicating circuit 4. A starting control for an AC. electric motordevice according to claim 1, wherein said synchronous indicating circuitincludes three serially connected transistors, one of said transistorsbeing turned on when one of said voltages across said thyristorswitching device is of a predetermined polarity prior to the others andbeing held in its turn-on state for a predetermined time interval, theother transistors being respectively tun-red on when said voltagesacross said thyristor switching device are respectively in apredetermined polarity.

i i 1% I!

1. A starting control for an A.C. electric motor device comprising: anA.C. electric motor device; an A.C. voltage supply for supplying voltageto said motor; a thyristor switching device having thyristors thereinand being connected between said motor device and said A.C. voltagesupply; a thyristor temperature rise anticipation circuit forintermittently turning off said thyristor switching device byanticipating a predetermined temperature rise of the thyristors providedin the thyristor switching device in response to an excessive motorcurrent during a starting of said motor; a thyristor temperature fallanticipation circuit for intermittently turning on said thyristorswitching device during the starting of said motor by anticipating apredetermined temperature fall of the thyristors provided in thethyristor switching device in response to the turn-off of the thyristorswitching device; a synchronous indicating circuit for detectingsynchronization between voltages across the said thyristor switchingdevice while the same is turned off, and, a synchronized connectingcircuit for enabling said thyristor switching device to turn on inaccordance with said thyristor temperature fall anticipation circuit. 2.A starting control for an A.C. electric motor device according to claim1, wherein said thyristor temperature rise anticipation circuitcomprises an integrating circuit which includes resistance andcapacitance elements therein; and, said thyristor temperature fallanticipation circuit comprises an integrating circuit which includesresistance and capacitance elements therein.
 3. A starting control foran A.C. electric motor device according to claim 1, wherein saidsynchronized connecting circuit comprises: a D.C. voltage supply; firstcircuit means being energized from said D.C. voltage supply in responseto an output from said thyristor temperature fall anticipation circuit;second circuit means for energizing the gate of the thyristors of saidthyristor switch; and, a switching transistor device for connecting saidfirst and second circuit means in accordance with an output from saidsynchronous indicating circuit.
 4. A starting control for an A.C.electric motor device according to claim 1, wherein said synchronousindicating circuit includes three serially connected transistors, one ofsaid transistors being turned on when one of said voltages across saidthyristor switching device is of a predetermined polarity prior to theothers and being held in its turn-on state for a predetermined timeinterval, the other transistors being respectively turned on when saidvoltages across said thyristor switching device are respectively in apredetermined polarity.